This invention relates to elastic fibers. In one aspect, the invention relates to bicomponent elastic fibers while in another aspect, the invention relates to biconstituent elastic fibers. In another aspect, the invention relates to bicomponent and biconstituent elastic fibers having a core/sheath construction. In yet another aspect, the invention relates to such fibers in which the polymer that forms the sheath has a lower melting point than the polymer that forms the core. In still another embodiment, the invention relates to methods of forming elastic cellulosic structures from a combination of cellulosic fibers and elastic bicomponent and/or biconstituent fibers having a core/sheath construction.
Cellulosic structures are known for their absorbency, and this property makes these structures useful in a wide variety of applications. Typical examples of such applications are diapers, wound dressings, feminine hygiene products, bed pads, bibs, wipes, and the like. The purpose of these products, of course, is to absorb and retain liquids, and the efficiency of these products in performing these tasks is determined, in large part, by their structure. U.S. Pat. Nos. 4,816,094, 4,880,682, 5,429,856 and 5,797,895 describe various such products, their construction and the materials from which they are made, and each is incorporated herein by reference.
Typically, absorbent cellulosic structures are made of materials that do not easily stretch. For example, cellulose fibers are, for all intent and purpose, inelastic and in many cellulosic structures, e.g. a diaper, they are bonded to one another in a relatively inelastic manner, e.g., through the use of a latex. Unfortunately, many of these structures require some degree of elasticity for reasons of comfort and use, e.g., a diaper conforming to the contours of the human body or a wipe having the touch and drape of cloth, and if the structure is not sufficiently elastic, gaps will form within it. Gaps reduce the absorbency of the structure by preventing the migration of the liquid to all parts of the structure.
Demand exists for better form-fitting absorbent products. This usually means that not only must the products have improved elasticity, but they must also be thin and light. Elasticity has been chased to date by adding to or replacing some of the cellulose fibers with an elastic fiber. For example, U.S. Pat. No. 5,645,542 to Anjur et al., the disclosure of which is incorporated herein by reference, describes absorbent products made from a wettable staple fiber (e.g., cellulose fiber) and a thermoplastic elastic fiber, e.g., a polyolefin rubber. However, the mere blending of staple fibers with elastic fibers often is not enough to obtain the full benefit of the elastic fiber without compromising the absorbency of the staple fiber. Cellulose fibers (the commonest of the staple fibers) tend to adhere to one another as opposed to adhering with an elastic fiber. As a result, unless a highly uniform mixture of the two fibers is formed during the construction of the absorbent structure, the two types of fibers tend to segregate and the benefit of the elastomeric fibers is reduced or lost.
Accordingly, the absorbent product industry has a continuing interest in the design and construction of absorbent products with improved elasticity without a compromise in absorbency. This interest extends to both the nature of the fibers from which the absorbent products are made, and the methods by which these absorbent products are constructed.
In one embodiment, the invention is a bicomponent fiber of a core/sheath construction in which the core comprises the thermoplastic elastomer, preferably a thermoplastic polyurethane (TPU), and the sheath comprises the homogeneously branched polyolefin. Preferably, the polymer of the sheath has a lower melting point than the polymer of the core, and more preferably the polymer of the sheath has a gel content of less than 30 percent.
In another embodiment, the invention is a biconstituent fiber in which one constituent comprises the thermoplastic elastomer, preferably a TPU, and the other constituent comprises the homogeneously branched polyolefin. Preferably, the constituent that forms the majority of the external surface of the fiber has a lower melting point than the other constituent, and preferably has a gel content of less than 30 percent.
In another embodiment, the invention is a blend of fibers (or simply a xe2x80x9cfiber blendxe2x80x9d) comprising (i) an elastic fiber comprising an elastic core and an elastic sheath, and (ii) at least one fiber other than the elastic fiber of (i). The core of the elastic fiber preferably comprises a thermoplastic elastomer, preferably a TPU, and the sheath of the elastic fiber preferably comprises a homogeneously branched polyolefin, more preferably a homogenously branched, substantially linear ethylene polymer. The polymer of the sheath has a melting point below the melting point of the polymer of the core, and preferably the polymer of the sheath has a gel content of less than 30 weight percent. The fiber of (ii) is essentially any fiber other than the fiber of (i), preferably a fiber of cellulose, wool, silk, a thermoplastic polymer, silica or a combination of two or more of these. In another embodiment of the invention, the fibers of (i) are melt bonded to the fibers of (ii), preferably by exposure to a temperature that is at or slightly below the melt temperature of both the fiber of (ii) and the polymer of the core of fiber (i) but above the melt temperature of the polymer of the sheath of fiber (i). In yet another embodiment of this invention, the melt bonded fiber blend is substantially free of any added adhesives, e.g., glue.
In another embodiment of this invention, the blends described in the preceding paragraph are used to build elastic, absorbent structures. Such structures include paper with elasticity, e.g., form-fitting labels, and the absorbent padding of a disposable diaper.
In another embodiment, the invention is a fabricated article comprising elastic fiber and a nonwoven substrate, the fiber comprising at least two elastic polymers, one polymer preferably a thermoplastic elastomer, more preferably a TPU, and the other polymer a homogeneously branched polyolefin, preferably a homogeneously branched, substantially linear ethylene polymer, in which the fiber is melt bonded to the nonwoven substrate in the absence of an adhesive. Exemplary fabricated structures of this embodiment include the leg cuffs, leg gatherers, waistbands and side panels of a disposable diaper.
In another embodiment of the invention, the ratio of nonelastic staple fibers, e.g., cellulose fibers, bonded to elastic fibers versus nonelastic staple fibers bonded to other nonelastic staple fibers, is increased by a method in which the elastic fiber is a hydrophobic fiber grafted with a hydrophilic agent, e.g., a polyethylene fiber grafted with maleic anhydride. In an extension of this embodiment, and in which the hydrophilic agent is an acid or an anhydride, e.g., maleic anhydride, once the agent is grafted to the fiber it is then reacted with an amine.
In another embodiment of the invention, for those nonelastic staple fibers that bind to one another due to hydrogen bonding, e.g., cellulose fibers, the ratio of nonelastic staple fibers bonded to elastic fibers versus nonelastic staple fibers bonded to other nonelastic staple fibers is increased by treating the nonelastic staple fibers, prior to or simultaneously with blending these fibers with the elastic fibers, with a debonding agent, e.g., a quaternary ammonium compound containing one or more acid groups. The debonding agent deactivates at least a part of the hydrogen bonding between the nonelastic staple fibers.
In another embodiment of the invention, blending of nonelastic staple fibers with elastic fibers is enhanced by blending the fibers in an aqueous media, preferably in the presence of a surfactant and with intense agitation. This procedure enhances the separation of the elastic fibers from one another, and thus makes each fiber more accessible for bonding with a nonelastic staple fiber. This method can be used alone or in combination with one or more other fiber separation embodiments of this invention.
In another embodiment of the invention, high intensity air mixing is used to separate elastic fibers from one another prior to blending with staple fibers. This technique also promotes separation of the elastic fibers from one another, and this, in turn, improves their accessibility for bonding with the staple fibers. This embodiment of the invention can also be used alone or in combination with one or more other embodiments of the invention.
The three fiber separation and the grafting embodiments described above are particularly useful in the construction of elastic absorbent structures such as diapers, wound dressings and the like.